1. Field of the Invention
The present invention relates to a circular waveguide antenna and a circular waveguide array antenna.
2. Description of the Related Art
Ordinarily, because reciprocal relationships are established at an antenna, transmission characteristics and reception characteristics are identical. Therefore, descriptions given hereafter are described for cases of transmission unless otherwise stated, and because cases of reception are the same, descriptions thereof will not be given.
In recent years, with the remarkable development of wireless communications technologies, there have been growing shortages of frequency bands to be assigned to various communication devices. In order to compensate for this, technological developments which are necessary to transfer effective utilization of frequencies to higher ranges have become an urgent matter. For example, millimeter waves, which have conventionally been used virtually only for basic research, have come to be used for highway transport systems (ITS: Intelligent Transport System). In the near future, as with household electronics, automobile companies in Japan, Europe, America, etc. can expect explosive growth in the use of millimeter wave-based communication devices.
In the field of millimeter-wave communications as mentioned above, obviously, it will be essential to adapt various components and apparatuses for millimeter waves. Among these, the one apparatus which is most important for millimeter-wave communications is the antenna. Without an antenna capable of transmitting and/or receiving millimeter-wave signals, millimeter-wave communications cannot be established. Currently, research institutions and manufacturers around the world who are participating in research and development of millimeter-wave communications are competing to develop millimeter-wave antennas with high levels of functionality. Hitherto, millimeter-wave antennas with various structures have been developed, and among these one millimeter-wave antenna with particularly excellent characteristics is the circular waveguide array antenna.
Next, an example of a previously known circular waveguide array antenna will be described. Firstly though, an example of a common circular waveguide antenna which structures a circular waveguide antenna array will be described.
The circular waveguide antenna is structured with a feeding portion and a radiating portion. There are various kinds of feeding portion, but the radiating portion is formed of a conductor in a tubular shape. A diameter and length thereof are determined by the wavelength employed, a state of matching with the feeding portion, and radiation directional characteristics. The higher an employed frequency is—that is, the shorter a wavelength λ is—the smaller the diameter of a tube of the radiating portion is, and the more difficult is machining of the feeding portion and the radiating portion.
FIGS. 30A and 30B show an example of structure of a previously known circular waveguide antenna. FIG. 30A is a perspective view and FIG. 30B is a sectional perspective view. A circular waveguide 31 is cut to a certain length, and is electrically connected with a dielectric sheet 32, which is provided with a conductive film, and earthed. A dielectric sheet 33 and the dielectric sheet 32 sandwich a stripline 34, which is a propagation path, and form the stripline 34. The stripline 34 has the function of propagating electric signals, extends to the middle of the circular waveguide 31, and structures the circular waveguide antenna.
A stripline distal end 36 of the stripline 34 is exposed at a central portion of the circular waveguide 31. An exposed length thereof and the diameter of the circular waveguide 31 determine impedance of the antenna. A dielectric exposure portion of the dielectric sheet 32 provided with the conductive film has the conductive film removed therefrom, to match a lower portion opening of the circular waveguide 31. The dielectric exposure portion covers the stripline distal end 36, and structures a feeding portion 37.
Ordinarily, electromagnetic waves radiate up and down from the stripline distal end 36. A cylindrical cavity 38 with a diameter the same as the circular waveguide 31 is formed in a conductor plate 35 such that it will not be the case that only half of the electromagnetic waves are irradiated from an upper portion opening of the circular waveguide 31. The cylindrical cavity 38 matches the lower portion opening of the circular waveguide 31 and is provided directly therebelow. A surface of the cylindrical cavity 38 is subjected to a surface treatment so as to be highly reflective of the electromagnetic waves that are employed.
A depth of the cylindrical cavity 38 is approximately a quarter of a wavelength in the guide λg for a central frequency of the employed frequency band. Accordingly, electromagnetic waves which radiate downward from the stripline distal end 36 are propagated a distance of λg/4, and completely reflected upon reaching a lower face of the cylindrical cavity 38. A phase inversion of 180° occurs thereat, and then the waves are again propagated the distance of λg/4 and return to the stripline distal end 36.
Thus, a propagation distance of the electromagnetic waves which radiate downward from the stripline distal end 36 is λg/2 (=λg/4+λg/4), and the phase inversion of 180° due to the total reflection corresponds to a further propagation of λg/2. Thus, the electromagnetic waves which are totally reflected at the lower face of the cylindrical cavity 38 and return therefrom are in phase with the electromagnetic waves which radiate upward from the stripline distal end 36, and efficient radiation from the upper portion opening of the circular waveguide 31 results.
Now, if a polarization plane of the radiated electromagnetic waves and a degree of stability are considered, a wavelength λ of the central frequency and a diameter a of the circular waveguide 31 are selected such that a propagation mode of the electromagnetic waves in the circular waveguide 31 is a basic mode TE11. In order to maintain the TE11 mode, the employed wavelength λ must be smaller than a cutoff wavelength λc (=3.412a) of the TE11 mode.
With such a structure, in accordance with machining of the stripline distal end 36, the circular waveguide antenna can be formed as a circular waveguide antenna for linearly polarized waves or a circular waveguide antenna for circularly polarized waves.
Further, the conventional circular waveguide antenna as described above may be formed in a single plate (see, for example, Non-patent Reference 1).
If such a circular waveguide antenna is plurally arranged to form an array device, a circular waveguide array antenna is obtained. If, for example, the antennas are arranged with equal spacings over a planar area, an antenna with radiation characteristics substantially equivalent to an aperture antenna with an aperture corresponding to the area of arrangement can be obtained.
Further, an array antenna is an antenna system in which a plurality of antennas are arranged in a pattern and which is capable of providing characteristics which cannot be provided by a single antenna. Further still, by controlling phases of the respective element antennas structuring an array antenna, it is possible to control directional characteristics of the overall antenna system, and thus it is possible to utilize the array antenna as a beam-scanning antenna without the main body of the antenna being mechanically moved.
As the name indicates, a circular waveguide array antenna is an array antenna in which a plurality of the conventional circular waveguide antenna are arranged in a certain pattern to serve as element antennas. The circular waveguide antennas are antennas in which the circular waveguides are cut off to certain dimensions and these are provided with excitation sections, and the cut-off openings serve as apertures.
A desired electric field distribution in a certain region can be obtained in accordance with the dimensions and arrangement of the circular waveguide antennas. For example, a plurality of circular waveguide antennas are two-dimensionally arranged in a planar region, and an electric field distribution with uniform direction, phase and amplitude can be obtained. Radiation characteristics of such an antenna are in theory substantially the same as radiation characteristics of an aperture antenna with a uniform electric field distribution, but such an antenna is more excellent than an aperture antenna in terms of freedom of structure and uniformity of the electric field distribution.
In a conventional two-dimensional array antenna, the element antennas which structure the array antenna are connected with a signal source by linked propagation paths, and the propagation paths are connected with the signal source or a feeding port of the array antenna.
At the same time, the propagation paths fulfill the role of phase devices, and lengths of the propagation paths from the signal source to the respective element antennas determine phases of the electromagnetic waves which are radiated from the respective element antennas, which affects radiation characteristics of the array antenna as a whole. Depending on the case, when phase adjustment is necessary, phase devices may be further added in series with the propagation paths (see, for example, Patent Reference 1).
Next, an example of a previously known circular waveguide array antenna in which the circular waveguide antenna described above is plurally arranged as array elements will be described.
FIG. 31A is a perspective view and FIG. 31B is an exploded perspective view of the above-mentioned circular waveguide array antenna.
A radiation plane of the antenna is a circular waveguide plate 41 which is machined with circular waveguides, which act as upper portion openings of the array elements, in a square region with equal spacings. Openings 42 of the array elements and screw holes 43, which are required when assembling the antenna and when fixing the antenna to other apparatus, are formed in the circular waveguide plate 41.
At a rear side of the circular waveguide plate 41 from the radiation plane, a stripline circuit sheet 44, an electromagnetic wave reflection plate 45 and a feeding port plate 46 are provided, and are respectively electrically connected by bolts or the like. The stripline circuit sheet 44 is for feeding the circular waveguides. The electromagnetic wave reflection plate 45 returns electromagnetic waves, which are radiated from distal ends of (feeding) striplines 47 of the stripline circuit sheet 44 when the same are feeding the openings 42 of the array elements, to the upper portion openings. The feeding port plate 46 feeds a common terminal of the feeding striplines.
At the stripline circuit sheet 44, the striplines 47 provided thereon are sandwiched by sheets of dielectric material, and the circular waveguide plate 41 and the electromagnetic wave reflection plate 45 are not directly connected electrically. An upper dielectric sheet, which is at lower portions of respective circular waveguides 410 of the circular waveguide plate 41, is removed at portions with dimensions exactly the same as the circular waveguides 410, to expose just distal end portions of the striplines 47, and electromagnetic waves can be radiated with ease.
This is a structure matching the circular waveguide antenna described with FIGS. 30A and 30B, and is a structure such that feeding terminals of all the striplines 47, which are guided to the lower portions of all the circular waveguides 410 of the circular waveguide plate 41, are structured as described in Non-patent Reference Document 1. The feeding terminals branch from a certain common terminal. Viewed from this common terminal, the feeding terminals are structured with the same physical conditions, and the polarization planes, electrical powers and phases of the electromagnetic waves radiated from the respective feeding terminals of the striplines 47 are the same. The common terminal receives electricity fed through coaxial wiring through a feeding port of the feeding port plate 46.
At the electromagnetic wave reflection plate 45, non-penetrating cylindrical cavities 48 with positions and diameters the same as all the circular waveguides 410 of the circular waveguide plate 41 are machined into the electromagnetic wave reflection plate for reflecting the electromagnetic waves that radiate downward from the feeding terminals of the striplines 47 back upward. Respective depths of the non-penetrating cylindrical cavities 48 are approximately a quarter of a wavelength in the tubes λg. Here, floor faces of the non-penetrating cylindrical cavities 48 must be treated so as to be completely flat and reflect the electromagnetic waves well. The plate 46 is a plate including an antenna feeding port 49, and is electrically connected with other apparatus through the feeding port 49. When a high-frequency signal is fed to this feeding port 49, the common terminal of the striplines 47 at the stripline circuit sheet 44 is structured to receive the high-frequency signal and equally distribute the high-frequency signal to the feeding terminals of all the striplines 47.
FIGS. 32A and 32B are detailed views of the circular waveguide plate 41 of FIGS. 31A and 31B. FIG. 32A is a perspective sectional view of the circular waveguide plate 41 and FIG. 32B is a sectional view seen in front elevation. The circular waveguide plate 41 is a conductor plate with a thickness of several mm, in which cylindrical holes are machined in a square region at a central portion with diameters determined in consideration of the electromagnetic wave propagation mode TE11, to structure the openings 42 of the array elements. The openings 42 of the array elements are cylindrical through-holes, and are orthogonal to the circular waveguide plate 41.
Here, a reason for selecting circular waveguides is that it is possible to form the tubular holes with high machining accuracy and with ease by drilling or the like. However, in the propagation mode TE11, an electric field distribution at the upper portion openings of the array elements is by no means an optimal electric field distribution.
The conventional circular waveguide array antenna shown in FIGS. 31A and 31B is, in a sense, an array in which the circular waveguide antenna shown in FIGS. 30A and 30B is structured with a compact form. When machining so as to form the circular waveguide antennas which will be array elements, it is possible to realize high arrangement accuracy of the array elements, high dimensional accuracy and easy machining, all at the same time.
In order to form the circular waveguide array antenna as a circular waveguide antenna for linearly polarized waves or a circular waveguide antenna for circularly polarized waves, the array elements structuring the antenna are set as circular waveguide antennas for linearly polarized waves or circular waveguide antennas for circularly polarized waves (see, for example, Non-patent References 2 and 3).
Patent Reference 1: Japanese Patent Application Laid-open (JP-A) No. 2000-353916 (paragraphs 0014 to 0019 and FIG. 1)
Non-patent Reference 1: Seiji Nishi, Yozo Shoji and Hiroyo Ogawa: “Millimeter-Wave Ad-Hoc Wireless Access System II: (7) 70 GHz Circular Polarization Antenna”, Technical Digest, 5th Topical Symposium on Millimeter Waves TSMMW2003, pp. 65-68, March 2003, Kanagawa, Japan.
Non-patent Reference 2: Seiji Nishi, Kiyoshi Hamaguti, Toshiaki Matui, Hiroyo Ogawa: “A Wireless Video Home-Link Using 60 GMHz Band: A Proposal Of Antenna Structure”, Proc., 30th European Microwave Conference, Volume 1, pp. 305-308, October 2000, Paris, France.
Non-patent Reference 3: Seiji Nishi, Kiyoshi Hamaguti, Toshiaki Matui, Hiroyo Ogawa: “Development of Millimeter-Wave Video Transmission System II: Antenna Development”, Technical Digest, 3rd Topical Symposium on Millimeter Waves TSMMW2001, pp. 207-210, March 2001, Kanagawa, Japan
There is no reason not to consider Non-patent Reference 1 or the conventional circular waveguide antenna shown in FIGS. 30A and 30B as, in a sense, conical horn antennas. That is, a circular waveguide antenna is a conical horn antenna with an opening angle of 0°. At the circular waveguide of such a conventional circular waveguide antenna, an opening 20 for radiating electromagnetic waves is ordinarily employed without alteration from a cut-off circular waveguide. Thus, there has been a problem in that it is not in any way possible to obtain optimal radiation characteristics.
Moreover, the conventional circular waveguide antenna illustrated in Non-patent Reference 1 has had a problem in that reflection loss characteristics of the antenna are not good and radiation gain is low.
Furthermore, the conventional array antenna illustrated in Patent Reference 1 is an antenna in which a number of electromagnetic horn elements is reduced and radiation characteristics are improved, but has had a problem in that the forms of the electromagnetic horn elements are large and it is not possible to make them small. Furthermore, there has been a problem in that forms of the electromagnetic horn elements that would maximize radiation gain have not been clear.
Moreover, the conventional circular waveguide array antennas illustrated in Non-patent References 2 and 3 have had a problem in that reflection loss characteristics of the antennas are not good and radiation gains are low.
Further, in high-frequency circuits, characteristics of devices may deteriorate due to adverse effects of reflected waves, and operations may cease. If reflected waves are to be blocked, it is necessary to provide matching circuits and cutoff filters or the like at the feeding terminals. If, for example, a matching circuit and a filter, or an isolator or the like, are provided prior to a feeding port, then it is necessary to adjust the impedance of the antenna. Consequently, there has been a problem in that antennas are larger and costs are higher.
Accordingly, provision of a low-cost, compact circular waveguide array antenna which can enable both ameliorated antenna reflection loss characteristics and improvements in radiation characteristics, particularly radiation gain, has been desired.